Orthogonal frequency-division multiple access (OFDMA) is a radio modulation scheme for wireless networking. Like its predecessor, orthogonal frequency-division multiplexing (OFDM), OFDMA slices a channel's bandwidth into subcarrier frequencies over which portions of a data stream may be carried in parallel. Since the subcarriers are independent transmission streams, their throughput may independently vary. This allows OFDMA to more rapidly modulate the higher quality subcarriers and degrade gracefully when some subcarriers succumb to excessive noise or fade. OFDMA improves upon OFDM by letting different users simultaneously transmit on separate subcarriers. The spatially variable nature of noise and fade means that the observations by different users will not agree as to which subcarriers are higher quality. When these observations are conveyed to an OFDMA base station, the base station may better match frequencies with users, thereby achieving diversity gain.
An OFDMA resource scheduler, typically implemented within a base station, multiplexes users across time and frequency. The resource scheduler accomplishes media access control by assigning to users portions of time-frequency known as resource blocks. If a user is assigned too many resource blocks for a given time, the user's peak power will be too high, which is a problem that affected OFDMA's predecessor, OFDM. If a user is assigned too few resource blocks at a time, the user's transmission will take longer and possibly jeopardize quality of service or cause an application timeout. If a resource scheduler divides the frequencies amongst too many simultaneous users or grants a user multiple frequencies that are not contiguous, interference may arise between subcarriers.
Although OFDMA schedulers are maturing, a new class of OFDMA terminal is emerging in a potentially disruptive way. These are the machine-to-machine (M2M) devices deployed out in the wild and programmed to routinely make contact with back-office servers and without human interaction. Examples of M2M devices include vending machines, security alarms, vehicle fleets, utility meters, and digital billboards. The applications and form factors of M2M devices are diverse, but their machine type communications (MTC) have consistent characteristics. In this sense M2M devices are akin to remote sensors. They tend to transmit more than receive. The transmissions are small and periodic. M2M devices have high tolerance for delays and expect little quality of service. Masses of M2M devices may be deployed by one owner who must be more price sensitive than an owner of an individual user equipment such as a smart phone. Due to these factors, the characteristics of M2M transmissions are typically the opposite of communications by human users. An OFDMA resource scheduler that treats user communications and M2M communications as being identical will likely not achieve an optimal assignment of resources and risks degrading the quality of service of more important or more profitable communications.